Though drug trafficking was first detected on federal lands in the mid-1990s, the activity has since spread to 20 states and 67 national forests. Traffickers are planting illicit crops on public land, destroying and defiling pristine wilderness while creating risks for hunters and other parkgoers.

A raid in August in Wisconsin's Chequamegon-Nicolet National Forest resulted in the seizure of more than 8,000 marijuana plants and seven arrests, at least six tied to drug-plagued Mexico.

Benjamin Wagner, the U.S. attorney for the Eastern District of California, which has been dealing with the problem for years, says it makes sense to drug traffickers "to move marijuana cultivation ... closer to your point of sale."

In August, Operation Mountain Sweep targeted marijuana crops on public lands in seven Western states, including California. About 578,000 plants worth more than $1 billion were eradicated, Wagner says. "The vast majority" of those arrested were "illegal aliens from Mexico or people here of Mexican extraction."

The activity comes as a survey last week shows a robust market for marijuana in the USA among young people: More teens smoke marijuana monthly than cigarettes.

The relaxation of marijuana laws - some states allow medical use, and Washington and Colorado voters legalized possession of small amounts last month - "certainly doesn't help the situation," Wagner says. "It creates an environment in which law enforcement gets mixed signals."

Federal agencies declined to comment on the influx of growers in U.S. parks. A U.S. Senate panel took up the issue last year.

"Violent transnational criminal organizations are exploiting some of our most pristine public and tribal lands as grow sites for marijuana," R. Gil Kerlikowske, director of the Office of National Drug Control Policy, told the panel in December 2011.

Drug traffickers were first detected on Forest Service land in California in 1995, David Ferrell, the Forest Service's law enforcement and investigations director, told the panel. From 2005-2010, he said, undocumented immigrants tended 1,607 cultivation sites in national forests.

Nationwide, 6.2 million marijuana plants found in outdoor plots were destroyed in 2011, more than double those eradicated in 2004, Drug Enforcement Administration data show.

The problem isn't confined to Western states. A site in a Michigan forest, where 3,000 plants were seized in 2011, was linked to Mexican drug groups. Eleven Mexican nationals were indicted in connection with the seizure in 2010 of more than 2,500 plants in rural Ohio.

Warren Eth, a Miami lawyer and expert on the trend, calls it "a huge problem," as growers cut down trees and pollute streams with chemicals.